The electrophotographic process is a method for obtaining a printed matter, the method including:    uniformly charging a surface of an image carrier such as a photosensitive member (charging step),    forming an electrostatic latent image on the surface of the image carrier by exposure (exposing step),    developing the formed electrostatic latent image using a developer containing colored resin particles (developing step),    transferring the developer image onto a recording medium such as paper or a plastic film (transferring step), and    fixing the transferred developer image on the recording medium (fixing step). In this case, the developer is broadly classified into a dry developer that uses colored resin particles composed of a material containing a coloring agent such as a pigment and a binder resin in a dry state and a liquid developer in which colored resin particles are dispersed in an electrical insulating liquid serving as a carrier liquid.
In recent years, color printing and high-speed printing have been increasingly demanded for image forming apparatuses that use an electrophotographic process, such as copiers, facsimiles, and printers. Since high-resolution and high-quality images are required in color printing, a developer that can form a high-resolution and high-quality image and is applicable to high-speed printing is needed.
A liquid developer is known as a developer having an advantage concerning the reproduction of color images. In such a liquid developer, the aggregation of colored resin particles does not readily occur during the storage, and thus a minute toner can be used. Therefore, such a liquid developer tends to provide excellent characteristics in terms of reproduction of thin-line images and tone reproduction. High-quality and high-speed digital printers that use an electrophotographic technique with a liquid developer and have such excellent characteristics have been enthusiastically developed. Under these circumstances, development of a liquid developer having better characteristics has been required.
There has been known a liquid developer in which colored resin particles are dispersed in an electrical insulating liquid such as a hydrocarbon organic solvent or a silicone oil. However, if the electrical insulating liquid remains on a recording medium such as paper or a plastic film, the image quality may considerably degrade and thus the electrical insulating liquid needs to be removed. The electrical insulating liquid is removed by a typical method in which the electrical insulating liquid is volatilized by adding thermal energy. However, such a method is not necessarily preferred in view of environment and energy saving because an organic solvent vapor may be emitted to the outside of the apparatus and a large amount of energy is required.
As a countermeasure, a method for curing an electrical insulating liquid through photopolymerization has been proposed. A photo-curable liquid developer is composed of a monomer or an oligomer having a reactive functional group as the electrical insulating liquid and also contains a photoinitiator dissolved therein. Such a photo-curable liquid developer is cured by reacting the reactive functional group through irradiation with light such as ultraviolet rays and thus is applicable to high-speed printing. Such a photo-curable liquid developer has been proposed in PTL 1. PTL 1 describes an acrylate monomer such as urethane acrylate as an example of the monomer having a reactive functional group.
However, an acrylate monomer has low volume resistivity and tends to decrease the potential of an electrostatic latent image in a developing step, which may cause difficulty in achieving a high image density and image blur (formation of an image with poor sharpness).
PTL 2 proposes that a curable liquid vehicle within a particular viscosity range and a particular resistance range is used as a curable electrical insulating liquid. Examples of the curable liquid vehicle include an epoxy compound, vinyl ether, and cyclic vinyl ether.